Many different types of memory exist to store data for computers and similar type systems. For example, random access memory (RAM), dynamic random access memory (DRAM), static random access memory (SRAM), read only memory (ROM), programmable read only memory (PROM), electrically programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM) and flash memory are all presently available to accommodate data storage.
Each type of memory has its own particular advantages and disadvantages. For example, DRAM and SRAM allow individual bits of data to be erased one at a time, but such memory loses its data when power is removed. EEPROM can alternatively be easily erased without extra exterior equipment, but has reduced data storage density, lower speed, and higher cost. EPROM, in contrast, is less expensive and has greater density but lacks erasability.
Flash memory, has become a popular type of memory because it combines the advantages of the high density and low cost of EPROM with the electrical erasability of EEPROM. Flash memory can be rewritten and can hold its contents without power, and thus is nonvolatile. It is used in many portable electronic products, such as cell phones, portable computers, voice recorders, etc., as well as in many larger electronic systems, such as cars, planes, industrial control systems, etc. Flash memory is generally constructed of many memory cells where single bits of data are stored in and read from respective memory cells.
Individual memory cells are generally organized into individually addressable units or groups, which are accessed for read, program, or erase operations through address decoding circuitry. The individual memory cells are typically comprised of a semiconductor structure adapted for storing a bit of data. For instance, many conventional memory cells include a stacked gate metal oxide semiconductor (MOS) device, such as a transistor in which a binary piece of information may be retained. The memory device includes appropriate decoding and group selection circuitry, as well as circuitry to provide voltages to the cells being operated on.
The erase, program, and read operations are commonly performed by application of appropriate voltages to certain terminals of the memory cell. In an erase or write operation the voltages are applied so as to cause a charge to be removed or stored in the memory cell. In a read operation, appropriate voltages are applied so as to cause a current to flow in the cell, wherein the amount of such current is indicative of the value of the data stored in the cell. The memory device includes appropriate circuitry to sense the resulting cell current in order to determine the data stored therein, which is then provided to data bus terminals of the device for access by other devices in a system in which the memory device is employed.
The traditional memory cell generally has a source, a drain, and a channel in a substrate or P-well, as well as a stacked gate structure overlying the channel. The stacked gate may further include a thin gate dielectric layer (sometimes referred to as a tunnel oxide) formed on the surface of the P-well. The stacked gate also includes a polysilicon floating gate overlying the tunnel oxide and an interpoly dielectric layer overlying the floating gate. The interpoly dielectric layer is often a multilayer insulator such as an oxide-nitride-oxide (ONO) layer having two oxide layers sandwiching a nitride layer. Lastly, a polysilicon control gate overlies the interpoly dielectric layer.
In a NOR configuration, the control gate is connected to a wordline associated with a row of memory cells to form sectors of such cells. In addition, the drain regions of the cells are connected together by a conductive bitline. The channel of the cell conducts current between the source and the drain in accordance with an electric field developed in the channel by the stacked gate structure. Respective drain terminals of the transistors within a single column are connected to the same bitline. In addition, respective flash cells associated with a given bitline have stacked gate terminals coupled to a different wordline, while all the flash cells in the array generally have their source terminals coupled to a common source terminal. In operation, individual flash cells are addressed via the respective bitline and wordline using the peripheral decoder and control circuitry for programming (writing), reading or erasing functions.
By way of further detail, the single bit stacked gate flash memory cell is programmed by applying a relatively high voltage to the control gate and connecting the source to ground and the drain to a predetermined potential above the source. A resulting high electric field across the tunnel oxide leads to the Fowler-Nordheim tunneling phenomena. During this process, electrons in the core cell channel region tunnel through the gate oxide into the floating gate and become trapped in the floating gate since the floating gate is surrounded by the interpoly dielectric and the tunnel oxide. As a result of the trapped electrons, the threshold voltage of the cell increases. This change in the threshold voltage (and thereby the channel conductance) of the cell created by the trapped electrons is what causes the cell to be programmed.
In order to erase a typical single bit stacked gate flash memory cell, a relatively high voltage is applied to the source, and the control gate is held at a negative potential, while the drain is allowed to float. Under these conditions, a strong electric field is developed across the tunnel oxide between the floating gate and the source. The electrons that are trapped in the floating gate flow toward and cluster at the portion of the floating gate overlying the source region and are extracted from the floating gate and into the source region by way of Fowler-Nordheim tunneling through the tunnel oxide. As the electrons are removed from the floating gate, the cell is erased.
For a read operation, a certain voltage bias is applied across the drain to source of the cell transistor. The drain of the cell is the bitline, which may be connected to the drains of other cells in a byte or word group. The voltage at the drain in conventional stacked gate memory cells is typically provided at between 0.5 and 1.0 volts in a read operation. A voltage is then applied to the gate (e.g., the wordline) of the memory cell transistor in order to cause a current to flow from the drain to source. The read operation gate voltage is typically applied at a level between a programmed threshold voltage (Vt) and an unprogrammed threshold voltage. The resulting current is measured, by which a determination is made as to the data value stored in the cell.
A modern memory technology is dual bit memory, which allows multiple bits to be stored in a single cell. In this technology, a memory cell is essentially split into two identical (mirrored) parts, each of which is formulated for storing one of two independent bits. Each dual bit memory cell, like a traditional cell, has a gate with a source and a drain. However, unlike a traditional stacked gate cell in which the source is always connected to an electrical source and the drain is always connected to an electrical drain, respective dual bit memory cells can have the connections of the source and drain reversed during operation to permit the storing of two bits.
In a virtual ground type architecture, dual bit memory cells have a semiconductor substrate with implanted conductive bitlines. A multilayer storage layer, referred to as a “charge-trapping dielectric layer”, is formed over the semiconductor substrate. The charge-trapping dielectric layer can generally be composed of three separate layers: a first insulating layer, a charge-trapping layer, and a second insulating layer. Wordlines are formed over the charge-trapping dielectric layer substantially perpendicular to the bitlines. Programming circuitry controls two bits per cell by applying a signal to the wordline, which acts as a control gate, and changing bitline connections such that one bit is stored by the source and drain being connected in one arrangement and a complementary bit is stored by the source and drain being connected in another arrangement.
A continuing trend in the electronics industry is to scale down electronic devices to produce smaller, yet more powerful devices (e.g., cell phones, digital cameras, etc.) that can perform a greater number of increasingly complex functions faster and with less power. To achieve this, semiconductors and integrated circuits (e.g., memory cells, transistors, etc.) utilized in these devices are continually reduced in size. The ability to “pack” more of these components onto a single semiconductor substrate, or a portion thereof (known as a die), also improves fabrication efficiency and yields. Accordingly, there are ongoing efforts to, among other things, increase the number of memory cells that can be fabricated onto individual semiconductor wafers (or die).
One technique to pack more memory cells/transistors into a smaller area is to form their structures and component elements closer together by utilizing photo-resist processes. However, photo-resist introduces well known problems, for example, in order to perform high energy implants the bitline width becomes larger than desired. Forming bitlines closer together, for example, shortens the length of a channel defined there-between and allows more devices to be formed in the same area however, the short channel width and the wide bitline cause certain phenomena to become more prevalent and have a substantial impact on device performance. For example, threshold voltage roll-off may become more pronounced as respective channel lengths are decreased. In particular, a threshold voltage (Vt), or voltage at which a transistor or memory cell “turns on” or becomes active to store and/or erase a bit of data (e.g., a charge), may vary (e.g., become reduced) as the respective channel length is decreased (e.g., source and drain regions are brought closer together). Additionally, the severity of threshold voltage roll-off may vary among different memory cells. In this manner, not all of a selected group of memory cells may be programmed/erased when the same or a similar voltage is applied to the cells. This could result in corrupt data and/or inaccurately stored or programmed data. Another issue is program disturb (TPD) which occurs, for example, when we program one device, another adjacent device also programs. This again results in inaccurately stored or programmed data.
Similarly, other issues, such as leakage currents, cross talk, etc., can also result from scaling. For example, isolating two bits or charges stored in a charge trapping layer becomes increasingly difficult as the channel length is decreased and the bits are brought closer together. In this manner, cross-talk and punch through currents can occur, the bits can contaminate one another and operations performed on one bit can affect the other bit (sometimes referred to complimentary bit disturb or CBD). Accordingly, it would be desirable to reduce feature sizes so as to increase packing density while mitigating the adverse affects that may result there-from, such as Vt roll-off, PD and CBD.